The invention relates to a switching arrangement or apparatus suitable for operating a high-pressure sodium discharge lamp by means of an adjustable power and provided with means for generating a control signal for controlling the adjustable power.
Such a switching arrangement is known from European Patent Application EP-A-240080, which corresponds to U.S. Pat. No. 4,952,864. In the known switching arrangement, the means for generating a control signal form a control signal dependent upon the lamp voltage. The adjustable power is controlled by means of the control signal in such a manner that the lamp voltage is constant by fair approximation. As a result, it is achieved that the colour temperature (T.sub.c) of the light emitted by the lamp is controllable to some extent and is subject to variations only to a comparatively limited extent.
Limitation of variation of colour temperature is of particular importance for high-pressure sodium lamps, which emit "white light". In general, in these lamps it holds for the colour temperature (T.sub.c) that T.sub.c &gt;2250 K. The range in the colour triangle within which the light of a high-pressure sodium discharge lamp is designated as "white", is limited by straight lines through points with coordinates (x, y): (0.400; 0.430), (0.510; 0.430), (0.485; 0.390) and (0.400; 0.360). The colour temperature may reach values of approximately 4000 K. in that case. Lamps of the kind described can be used to replace incandescent lamps.
A disadvantage of control of the colour temperature by means of the known switching arrangement is that the colour temperature depends only in part upon the lamp voltage. Especially sodium disappearance and hence variation of the amalgam composition of the lamp filling leads to variation of the colour temperature which cannot be controlled by means of control of the lamp voltage.